In storage systems such as distributed storage systems and so on, the access frequency to newly generated data is high. Such a storage system therefore performs replication, which ensures availability and reliability of data by replicating data and arranging replicated data in a plurality of storage nodes in a distributed manner.
However, replication has high storage overhead (low capacity efficiency). Therefore, when the access frequency becomes not so high, the storage system shifts from replication to redundant arrays of independent disks (RAID), which has low storage overhead.
A shift from replication to RAID involves not only reading data for parity calculation, but also transferring data between nodes, and hence might affect user operations. Further, in the course of shifting from replication to RAID, replication and RAID temporarily overlap. This increases the consumption of storage resources.
As a technique addressing these issues, there is known a disk array apparatus that is capable of changing the RAID level without transferring data between storage nodes according to a condition (see, for example, Japanese Laid-open Patent Publication No. 2004-94429).
However, a shift from replication to RAID still involves data transfer between storage nodes, and there is room to reduce the effects on user operations.